The main rotor blade failed in fatigue and separated from the helicopter under normal service loading. It is likely that the combination of a reduction in load transfer, brought about by the adhesive disbonding, and the significant residual stresses in the doubler skin following blade assembly, caused the onset of the fatigue failure. The pilot's injury pattern was consistent with the helicopter landing heavily on the left skid. Despite the use of the complete available harness and a helmet, the pilot was subjected to severe left lateral and vertical forces, as evidenced by the downward displacement of the seat pan and damage to the pilot's door. When the helicopter came to rest, the pilot was unable to extricate himself from the wreckage without assistance. The following TSB Engineering Branch report was completed: LP 190/97 - Main Rotor Blade ExaminationAnalysis The main rotor blade failed in fatigue and separated from the helicopter under normal service loading. It is likely that the combination of a reduction in load transfer, brought about by the adhesive disbonding, and the significant residual stresses in the doubler skin following blade assembly, caused the onset of the fatigue failure. The pilot's injury pattern was consistent with the helicopter landing heavily on the left skid. Despite the use of the complete available harness and a helmet, the pilot was subjected to severe left lateral and vertical forces, as evidenced by the downward displacement of the seat pan and damage to the pilot's door. When the helicopter came to rest, the pilot was unable to extricate himself from the wreckage without assistance. The following TSB Engineering Branch report was completed: LP 190/97 - Main Rotor Blade Examination A main rotor blade (green) failed in fatigue, likely because of non-conforming doublers, and separated from the helicopter. Severe vibration from the loss of the main rotor blade resulted in separation of the tail boom. After the tail boom separated, the pilot was unable to maintain control of the helicopter, and it landed heavily.Findings A main rotor blade (green) failed in fatigue, likely because of non-conforming doublers, and separated from the helicopter. Severe vibration from the loss of the main rotor blade resulted in separation of the tail boom. After the tail boom separated, the pilot was unable to maintain control of the helicopter, and it landed heavily. The pilot lost control of the helicopter when a main rotor blade failed in fatigue and separated from the helicopter, resulting in a severe vibration causing the tail boom to separate.Causes and Contributing Factors The pilot lost control of the helicopter when a main rotor blade failed in fatigue and separated from the helicopter, resulting in a severe vibration causing the tail boom to separate. Examination of the failed main rotor blade parts by the manufacturer resulted in the issuance of two Mandatory Service Bulletins (MSB); SB 369D-194, on 24 December 1997, requiring visual inspection of the root fitting area at 25-hour intervals on blades that have 1,500 or more hours of operation. SB369D-195R1, on 23 January 1998, requiring visual inspection of the root fitting area at 25-hour intervals on specific model and serial number blades that have operated 600 or more hours. The FAA issued two priority letters: AD 98-01-13, on 31 December 1997, mandating compliance with the inspection requirements of SB 369D-194. AD 98-03-15, on 29 January 1998, mandating compliance with the inspections requirements of SB 369D-195R1.Safety Action Examination of the failed main rotor blade parts by the manufacturer resulted in the issuance of two Mandatory Service Bulletins (MSB); SB 369D-194, on 24 December 1997, requiring visual inspection of the root fitting area at 25-hour intervals on blades that have 1,500 or more hours of operation. SB369D-195R1, on 23 January 1998, requiring visual inspection of the root fitting area at 25-hour intervals on specific model and serial number blades that have operated 600 or more hours. The FAA issued two priority letters: AD 98-01-13, on 31 December 1997, mandating compliance with the inspection requirements of SB 369D-194. AD 98-03-15, on 29 January 1998, mandating compliance with the inspections requirements of SB 369D-195R1.